Friday, August 22, 2014

Phase Change Storage with the Cryo-Compression Gas Turbine

   Thermodynamic cycle efficiency of an expansion engine is a function of the temperature difference between source and sink relative to the temperature of either. Heat sink energy is stored in a cryogenic phase change medium, just as heat source energy is stored in fuel, a phase change medium or the surrounding environment, The Cryo-GT described here operates with “source plus sink storage”. It should not be confused with a dual fuel engine. Storage density compared to that of a Li-ion battery ranges from approximately equal with only liquid air to 16 times with fuel plus liquid air. Source temperature may range from ambient atmosphere in a fuel-less engine to approximately 1630 F (890 C) in a fired stainless steel engine without blade cooling. A liquid air sink operates between the liquid phase at - 315 F (- 190 C) and solid phase at - 350 F (- 210 C). The gas turbine is selected as the base line engine for source plus sink storage because of the simplification afforded by external compression and associated capability to recover maximum deceleration energy in hybrid vehicle application.
   Phase change storage is gaining acceptance as indicated by an operating 300 kW pilot plant and a grant for a liquid air vehicle engine in the UK [1]. This technology has also been licensed to General Electric Oil and Gas for use with GE peaking turbines. A “liquid nitrogen economy” was proposed in 1974 [2] and some high pressure engines with cryogenic compression were built and tested, including a fired turbine [3] and two fuel-less reciprocating engines [4, 5].

   The Cryo-GT is more efficient and has application to a wider range of vehicle size than the engines of the UK program. The following tabulation compares operating (no) and total efficiency (nt) of the Cryo-GT with normally aspirated engines, where (nt) includes air liquefaction and fuel processing.
                             Cryo-GT      Otto      Micro-GT      Adv- GT 
                            (no)   (nt)    (no)   (nt)  (no)   (nt)      (no)   (nt)
Compact Car 
@50mph               51    19      18     14     --       --         --      --
Distrib Elec           74    43       --       --     28      22       --      --
Central Elec          82    25       --       --      --      --        40     29
   Tank-to-wheel efficiency does not take into account the renewable energy used for air liquefaction, nor does it account for fuel refining, distribution and exploration. Complete evaluation of source plus sink storage must include the impact of reduced fuel consumption on alternate fuels delivery, emissions and handling safety.

   Liquefied air is supplied to the Cryo-GT from advanced liquefiers driven by station off-peak and various sources of renewable energy. Advantages over battery storage include;
* long service life with no disposal requirement and no toxic or limited resources,
* consistent efficient performance,
* universal availability ranging from distributed generation to central station capacity and,
* low weight and capital cost in a well developed technology.

   Implications of low Cryo-GT fuel consumption on emissions and fuel selection with respect to safety and cost are profound. The fuel-less and fired Cryo-GT are both more efficient than a fuel cell, do not require ultra-high purity, and can be implemented years ahead. The Cryo-GT uniquely meets the defining requirement of economic liquid air consumption, as the ratio of liquid air to working fluid decreases with decreasing pressure ratio. The cycle may be described as a Brayton cycle, modified to include quasi-isothermal compression. Injection of liquid air into regeneratively cooled intake air reduces compression work from 55 % to 15 % of turbine output. The portion of liquid air in the working fluid is calculated by dividing compression work, according to the quasi-isothermal “Tds equation”, by latent heat of the liquid air.

    Cryo-GT is potentially a universal prime mover. Advantages of the Cryo-GT include nearly ambient exhaust temperature and capability to burn a variety of fuels or run on recovered heat. Service life is very long with high reliability and low emissions, maintenance and weight. Heat exchanger frost problems experienced with the early Cryo cars [4. 5] are avoided with an internally heated cryo-regenerator.  Low compression extends high efficiency to small engines. Design engine speed in a vehicle at 50 mph is 50,000 rpm, one-half as for a micro-turbine.
   Three optional features proposed by the author of this paper provide enhanced performance of the Cryo-GT; exhaust recirculation and heat recovery by an auxiliary jet-compressor/rotary regenerator, a brake driven heat of fusion sink, and cogeneration of fuel using heat rejected in the liquefaction process. The jet-compressor/rotary regenerator increases regenerator effectiveness in low pressure applications while reducing consumption of liquid air, as well as heat exchanger cost. Jet drive by high motive pressure of the liquefied air recirculates a portion of turbine exhaust to the combustor. Improvements to gas turbine heat recovery include reduced surface area, leakage and flow mal-distribution. The heat of fusion sink extends liquid air range of a vehicle by on-stream liquefaction of regeneratively cooled working fluid returning to the cryogenic compressor. It works by alternately melting solidified air during absorption of compression heat and re-solidifying the fluid by vacuum induced by a brake driven circulator. The sink reduces air liquefaction energy to about 25 % as for advanced air liquefiers. Finally, sufficient low pressure steam, generated by liquefier cooling water, is available to bio-fuel gasifiers for cogeneration of various fuels.
   Liquid air production requires the equivalent of 40 % of Cryo-GT output, however there are strong mitigating factors;
* low fuel consumption promotes use of renewable fuel,
* regeneratively cooled compressor intake air is pre-cooled for on-stream re-liquefaction by a 
   heat of fusion sink (thermal battery),
* rejected liquefier heat provides space heating and cogeneration of fuel,
* low cost station off-peak and recovered energy, including solar and building amplified wind, 
   drives liquefaction with uniform geographical distribution,
* low cost liquid air at 3 c/lb [1 ], and
* liquefier technology is advancing with the developing liquefied natural gas industry.

   Liquid air is selected over liquid nitrogen for compression coolant in the Cryo-GT for safety reasons. Problems associated with leakage of oxygen enriched air due to liquefaction are more manageable than potential asphyxiation in the event of liquid nitrogen leakage [1]. In addition, Cryo-GT uses a minimal inventory of light compressed natural gas (MW = 19.5) or hydrogen (MW = 2), which can by up-vented away from liquid air in the less likely event of simultaneous leakage. Furthermore, ignition is not an issue in fuel-less application.

   The following Table* presents weights, costs and other data for Cryo-GT compared to an Otto cycle engine and an electric motor/battery in a 3000 lb car with frontal area = 24 ft2, drag coefficient = 0.29, developing 11 HP (8.2) kW at 50 mph.

 Engine    fuel  lqa  batt  eng  range fill/chg lqa  fuel   life  eng/batt
                  lb     lb     lb     lb    mi      min   ¢/mi  ¢/mi  Khr   ¢/mi
CryoGT    18     85     50  100  300      2     1.0     2.5   20     0.3
       “         --    300   270  120  300      5    3.5      --      30    0.3
Otto          70     --      50   200  400      2      --    11.7     4      2.5
Otto-hyb   64     --    180   200  540      2      --      7.9     4      4.5
All elec     --      --     600  250  100   480     --       --       2     6.0 
1 CNG @100 atm.
2 wgt. (lb) turbine 25, regenerator 30, reheater 20, heat sink 30, comp 15 = 120

   The following Table* presents lqa/nat gas mass ratio, cost compared to normally aspirated stationary GT’s with 40 Khr life at design point press. ratio (Pr) and turbine inlet gas temp. (T).                         
          Power      Pr      T      lqa/nat gas      lqa      fuel      plant
                mW        --      F              --          ¢/kWhr  ¢/kWhr  ¢/kW
CryoGT  0.1          3   1630          39            3.0        4.0       5.5  
MicroGT 0.1          3   1630          --              --       10.5        8.0
CryoGT 100        18   2900           49            3.2       3.6        1.5
AdvGT  100       18   2900            --             --         7.0        2.0
*Assumed data are:
Costs: $4.00 US/gal of gasoline, 50 ¢ US/therm nat.gas, and 25 ¢ US/gal of liquid air
Cryo-GT: regenerator effectiveness = 95 % and turbine and compressor efficiency = 85 %
Fuel preparation = 25 % of higher heating value, and air liquefaction fig. of merit = 0.5

   In conclusion, source plus sink storage with the Cryo-GT prime mover should be considered as a viable expedient towards a clean energy future based on the following advantages;
* highest attainable operating efficiency, sustained over a wide load range,
* supercedes difficult high temperature solutions,
* low capital cost,
* builds on developed technology, and
* efficient storage provided by globally distributed atmosphere 

  1. Center for Low Carbon Futures, “Liquid Air in the Energy and Transport Systems”, 
      ISBN:978-0-9575872-2-9, 2013
  2. Kleppe, J. and Schneider, R., "A Nitrogen Economy", ASEE, 1974
  3. Kishimoto, K. et-al, “Development of Generator of Liquid Air Storage Energy System”,  
      Mitsubishi Heavy Industries Technical Review Vol. 35 No. 3, 1998
  4. Ordonez, C.,“Liquid Nitrogen Fueled, Closed Brayton Cycle Cryogenic Heat Engine”,   
      Energy Conversion and Management 41, 2000
  5. Knowlen, C. et al, "High Efficiency Energy Conversion Systems for Liquid Nitrogen
      Automobiles", University of Washington, SAE 981898, 1998